1. Field of the Invention
The invention relates to electronic amplifiers and, in particular, to a transresistance amplifier having a gain stage in its feedback path and a switched capacitor load, the tranresistance amplifier being particularly adapted for use in a radiation detection system.
2. Description of the Prior Art
The continuing goal of microelectronic designers is to increase the number of microelectronic circuits formed on a semiconductor chip of decreasing size, the circuits and their interconnections being manufactured simultaneously. A concurrent goal is to reduce the power dissipation of the chip.
In many types of signal processing systems, such as monolithic infrared focal plane signal processing systems used in space detection applications, for example, it is required that each component in the system be designed to optimize overall performance while reducing system power dissipation. In the above mentioned system, the signal processing chip is intended to process the output of a plurality of independent photovoltaic infrared detectors. The output of each detector is an electric current which must be converted into a voltage by a transresistance amplifier connected thereto, the voltage output of the amplifier being applied to a high pass filter to remove signal information from stationary background sources. The output of the filter is coupled to external circuitry via a multiplexer and buffer. The design of the transresistance amplifier in this system is thus based in part on the characteristics of the detector and filter. Regarding the photovoltaic detector, detector current is related to the incident light flux, the voltage bias across the detector and, to a lesser extent, the reverse saturation current.
In order to reduce detector noise, it is necessary to operate the detector with low (ideally zero) bias voltage. Operating the detector into a short circuit load would satisfy this requirement, the standard solution to such a problem being to use an operational amplifier as the load. However, this is difficult to achieve in practice because the available chip area in this application is limited and is unable to support an operational amplifier. Further, even if an operational amplifier could be utilized, it dissipates relatively large amounts of power. Therefore, it is desired to find a solution which permits the detector to be operated at a relatively low voltage level which remains essentially constant for a large variation in light flux while at the same time minimizing power dissipation and required chip area.
Other constraints placed on the design of the transresistance amplifier in addition to those due to the characteristics of the filter and detector include temperature (infrared detection applications require that the amplifier be in an extremely cool environment, (e.g., 77.degree. K.), dynamic range, actual transresistance, injection efficiency, and the type of active device (i.e., pMOS or nMOS) which can be used. Transresistance is a measure of the amount of AC output voltage generated for a given amount of AC input current, while injection efficiency is a measure of the amount of AC current provided to the detector load circuitry for a given amount of AC detector current.
A number of solutions have been proposed to provide a tranresistance amplifier which meets the above constraints. One such solution utilizes a gain stage in the gate electrode of a MOSFET transresistance amplifier, a second MOSFET with a resistive load being used as the gain stage. Although noise is reduced, the power dissipation due to the resistance load is relatively high, especially when considered in light of the relatively small improvement in injection efficiency provided by this arrangement. Another solution which is a variation on the above solution, utilizes a depletion mode MOSFET in place of the resistive load of the above arrangement, thus providing a high equivalent load resistance. However, power dissipation due to the resistance load is still higher than desired.
An article "Mosaic Focal Plane Methodologies" by Wong et al., Proceedings of the Society of Photo-Optical Instrumentation Engineers, Vol. 244, pp. 113-125 (1980) describes the use of Z-technology in fabricating mosaic sensor focal planes, Z-technology allowing increased signal processing integrated circuit area per detector channel. The article discloses a common gate reset transimpedance amplifier which band limits the noise of the detector and preamplifier and includes a synchronously clocked switched capacitor filter to achieve the desired function. As described in the article "MOS Switched-Capacitor Filters", Broderson et al., Proceedings of the IEEE, Vol. 67, No. 1, pp. 61-75 (January, 1979), the switched capacitor essentially functions as a resistor, the resistance value of which is related to the value of the switched capacitor and the switching frequency. Although this configuration performs satisfactorily, the detector voltage is not accurately controlled such that it is consistently maintained close to zero.
U.S. Pat. No. 4,100,407 to Takahashi discloses a circuit having a capacitor for comparing the output voltage of an operational amplifier, connected to a photosensor, with a reference voltage, and discharging means operable under the control of the capacitor to discharge the charge stored in a parasitic capacitor of the photosensor when the circuit is energized for operation; U.S. Pat. No. 4,320,347 to Hague discloses a switched capacitor employing an operational amplifier; and U.S. Pat. No. 3,988,689 to Ochi et al., U.S. Pat. No. 4,068,182 to Dingwall et al. and U.S. Pat. No. 4,255,715 to Cooperman disclose the use of switching capacitors. It should be noted that none of the patent references set forth hereinabove are concerned with providing a transresistance type amplifier which meet the design criteria set forth hereinabove.